1. Field of the Invention
The present invention relates to a rotary encoder having an airtight structure, and more specifically, it relates to a rotary encoder having a desiccant which is arranged in a hermetically sealed encoder body together with other components.
2. Description of the Related Art
In general, in hermetically sealed devices, condensation may occur in the devices due to a temperature difference between the external air of the devices and the internal air of the devices or water intruding into the devices. In view of this, conventionally, it is known to provide a desiccant in the devices for the purpose of absorbing the moisture entering the devices or preventing the occurrence of condensation in the devices (e.g., Japanese Patent Application Laid-open No. 2005-338764 or No. 2005-148035).
In particular, in a rotary encoder disclosed in Japanese Patent Application Laid-open No. 2005-148035, the encoder main body is hermetically sealed by a cover member but liquid can intrude into the encoder main body through a gap between a rotating axis portion of a rotary slit plate and a bearing portion supporting the rotating axis portion. Therefore, it is necessary to arrange a desiccant therein which is effective against the intruded liquid.
Moreover, it is possible to separately provide a moisture sensor or a condensation sensor in order to detect condensation. Japanese Patent Application Laid-open No. H11-2616 discloses a moisture sensor which detects the moisture and Japanese Patent Application Laid-open No. H05-40104 discloses a condensation sensor which detects condensation.
In the case where a rotary encoder is attached to, for example, a motor (servo motor) of an NC machining tool. However, the temperature and moisture in the encoder main body remarkably change due to heat of the motor or the encoder main body is exposed to liquid, e.g., cutting liquid, which tends to enter a clearance. If a sudden change in the temperature or moisture occurs or the cutting liquid enters into the encoder main body, even if a desiccant is provided in the hermetically sealed encoder main body as in the aforementioned prior art, the desiccant may not be able to absorb the liquid completely. In such cases, the encoder may be damaged by a large amount of liquid exceeding the moisture absorbing capacity of the desiccant.